Lasers comprise a preferred means of communication through free-space because they can transmit substantially more information over long distances in comparison to radio waves or microwaves, due to their much higher frequency and therefore significantly smaller beam divergence and greater bandwidth. However, laser communications traversing earth's atmosphere are degraded by ubiquitous turbulence. Lasers are affected more than lower frequency electromagnetic waves because atmospheric turbulence causes rapid changes in the density, and thus the index of refraction, across the path of the laser beam.
Due to the heterogeneous nature of turbulence, the rays composing the beam will encounter differing densities. This will cause their respective path lengths to the receiver to become unequal, resulting in differing phases and thus a distorted wavefront at the receiver. Furthermore, the received laser beam will no longer have the uniform amplitude that it had when emitted. The turbulence-induced distortion also increases the divergence of the beam, which reduces the signal-to-noise ratio (“SNR”) because it increases the portion of the laser beam that will fall outside of the receiver aperture.
Under operating conditions where the receiver noise is dominated by background light, the SNR is given by S2/n, where S and n are the number of photoelectrons per bit due to the signal and background, respectively. Background light, especially solar, represents a significant source of communications receiver noise. Reducing the receiver field-of-view lowers the background light, but it increases the difficulty of closing the communications link between the receiver and the transmitter, especially when the two are separated by a long distance or are translating relative to one another. One approach is to use a mechanical servo system to vary the direction of the receiver's field-of-view. However, such servo systems are expensive, bulky, heavy and mechanically complex. While certainly a consideration for even a terrestrial receiver, the size and weight become particularly critical when the receiver is to be placed in earth orbit or airborne.
Receiver noise sources other than background light, in particular detector thermal (Johnson) noise, further decrease the SNR of optical communications receivers. Heterodyne detection can overcome the noise introduced by receiver noise as well as background light, and provide quantum-limited detection, but its use in free-space communications has been rendered impossible or impractical due to pointing jitter and wavefront distortions of the received communications beam.
As encryption has become an increasingly important aspect of communications, the additional demands it places on laser communications have been brought to the forefront. Quantum encryption, which provides the ultimate encryption technique, requires distribution of a quantum key (“QKD”). QKD requires the receiver to detect and differentiate between individual photons. This, in turn, makes it necessary to eliminate or at least mitigate the adverse effects of atmospheric refraction and background light, while at the same time maximizing the field-of-view of the receiver to facilitate closure of the communications link.
One solution to the problem of atmospheric turbulence is described in U.S. Pat. No. 5,378,888, “Holographic System for Interactive Target Acquisition and Tracking,” issued to the present inventor. The foregoing reference uses real-time holography to generate a phase-conjugate laser beam that, after twice traversing the intervening turbulence, impinges the receiver aperture having the lateral cross section, the phase across its wavefront, and the angle of incidence that it would have had in the absence of turbulence. However, neither this reference nor any other prior art resolves the technical difficulties attendant to optical communications posed by receiver background light, detector noise, atmospheric turbulence, and the relative translation between the receiver and a transmitter; as well as provides the capability to detect and differentiate single photons for QKD under the foregoing conditions. The present invention fulfills these needs in the art.